Bioastronomy 2002: The Earth is a Tough Place

Seth Shostak, astronomer at the SETI Institute, is attending a week-long conference on life in the universe being held in Australia. He wraps up his coverage of the conference with this report.

Although bioastronomy is about life off of Earth, a lot of the really interesting research in this field concerns life on Earth. How did biology get underway on our planet, and what were the choke points that could have snuffed it out? Most high school history texts go back a few thousand years or so. Knowing the 4.5 billion years of history before that might give us insight into how life would fare on other worlds.

The first chapters of a really complete terrestrial history book are difficult to write. At the Bioastronomy 2002 conference, Australian researcher Mark Harrison pointed out that our understanding of early Earth is hindered by the fact that the geological record pretty much gives out once we go back farther than 4 billion years. There are no intact rocks from the convulsive, first 500 million years of our planets existence (a period appropriately known as the Hadean era). But Harrison has been able to analyze a rare, but ubiquitous silicon mineral known as zircon: found as small chunks of material that were sometimes produced during the Earths tumultuous youth. Zircons discovered in western Australia date from the Hadean era, and by studying the isotopes locked in these chips-off-very-old blocks, Harrison has concluded that continental crust began forming on our planet somewhere between 4.0 and 4.4 billion years ago. The continents are very old, in other words. More surprising is the fact that the oceans date back 4.3 billion years, or possibly more. Our atmosphere, which seems to have formed when clouds of gas billowed out of the Earths mantle, may have appeared quite early too: 4.4 billion years ago.

All that is truly ancient history. Other conference attendees talked about more recent events, including the havoc and destruction wreaked by rocks from space. The most famous of these, which occurred 65 million years ago when an object roughly 10 km in size plunged into the Yucatan, wiped out the dinosaurs and about half of all other species. It was, quite obviously, an important fork in the evolutionary road that allowed the emergence of clever mammals such as you and your friends.

Elisabetta Pierazzo, who works in Tucson, Arizona, has been trying to better understand the nature of this dramatic impact. She notes that the Yucatan landscape sported a 3 km thick layer of carbonates and other minerals that, when slammed with a massive rock, would cause the release of climatically important gasses (such as sulfate aerosols) into the atmosphere, cooling average temperatures world-wide for 3 to 5 years. In 1991, the Pinatubo volcano threw 20 billion tons of material into the air, and global temperatures dropped by about 0.5 C. Pierazzo estimates that the rock that slammed into the Yucatan 65 million years ago put several thousand times as much crud into the air, enough to cool temperatures world-wide by 5 to 10 C. Thats sufficient drop to destroy much of the plant life and, as consequence, starve most of the animals.

Most readers are undoubtedly familiar with killer rocks. But conference delegate Mike Rampino, from New York University, had another tale of destruction to tell. Rampino is keen on volcanoes, and points out that, while Pinatubo was an impressive eruption, there are super volcanic events that make it look like a firecracker. These super-eruptions, as theyre called, are hundreds of thousands of times larger in terms of the amount of material involved, sometimes spewing thousands of cubic kilometers of stuff into the sky. Imagine a cube of rock a dozen kilometers on a side, pulverized, heated and blown into the atmosphere. Rampino figures this could cause temperatures world-wide to drop by 10 C, for five years.

There was a volcanic eruption in Indonesia of this size a mere 73,000 years ago. Intriguingly, Rampino cites suggestive evidence of a reduction in the total human population from about a tenth of a million to 3 to 10 thousand individuals at this time and if this is true, then a super-eruption nearly extinguished Homo sapiens. And by the way, the frequency of such economy-size blowouts is estimated to be once every 50,000 years or so. Were overdue for a blast, in other words. Long-lived civilizations must clearly address the threat of super-eruptions, Rampino notes, presumably referring not only to us, but to any cosmic confreres.

Its a sobering thought. Sure, we think we know how to avoid the fate of the dinosaurs: we simply map the rocks that might hit Earth, and if one threatens, we send Bruce Willis to deflect it. But super-eruptions are not so easily dealt with. Perhaps well need an army of Willis to drill deep holes around potential monster volcanoes to release the pressure. Perhaps.

As these and other talks at Bioastronomy 2002 made clear, the history of Earth is filled with drama and surprise. Its exciting to be living in a time when at least some of the pages of this remarkable book are starting to be legible.